Detection of apoptosis by in situ labeling in pilomatricoma

Pilomatricoma is a benign epidermal appendage tumor with differentiation toward hair cortex cells. It is composed of varying percentages of basophilic cells, shadow cells, and transitional cells. Transitional cells seem to be apoptotic cells as evidenced by their morphological and ultrastructural characteristics. We investigated apoptosis in ten specimens of pilomatricoma by the transferase-mediated dUTP-biotin nick end labeling (TUNEL) technique. TUNEL-positive structures were detected in both transitional cell and amorphous debris zones, but not in either basophilic cell or shadow cell zones. In the transitional cell zone, apoptotic cells were found either in groups or bands, while in the amorphous debris zone, apoptotic bodies of varying sizes were seen diffusely. These findings indicate that transitional cells, due to the fact that they are dying cells, may not differentiate into the shadow cells. This view is supported by the facts that the transition of basophilic cells into the shadow cells is abrupt in some areas and that tumor cells, with or without ultrastructural characteristics of apoptosis, are found in the transitional cell zone. Furthermore, ultrastructural nuclear changes in shadow cells are different from those seen in apoptosis. Therefore, we propose that the name of transitional cells should be changed to ''condensed cells.'' Although it has been conventionally thought that basophilic cells in pilomatricoma differentiate directly and/or via transitional cells into shadow cells, we demonstrate that basophilic cells undergo two different processes. The first is differentiation directly into shadow cells, an the second is passage, via ''condensed cells'' (transitional cells), into the amorphous debris that results from tumor kinetics.